1. Field of the Invention
The invention pertains to optical systems for imaging objects and in particular to a method and apparatus for enabling more accurate focusing of such systems. Optical systems for producing an image are well known in the art. Such optical systems include projection systems and various types of cameras in which an object is imaged on an image plane.
2. Description of the Prior Art
In order to insure that optical systems accurately reproduce an object at the image plane, such systems are often provided with detection apparatus for determining whether the system is in focus or out of focus. In addition, such systems commonly include an adjustment mechanism for automatically or manually adjusting the optical system in order to establish and maintain focused condition. Typically, in manually focused camera systems the aforesaid detection apparatus is in the form of an optical range finder. The range finder presents the photographer with an image of the object whose condition is indicative of the focusing condition of the optical system of the camera. Thus in one type of a conventional range finder, the clarity of the range finder image is the condition which must be ascertained. In this type of range finder, microprisms may be provided to emphasize the out of focus condition. Thus the image appears either blurred, when the optical system of the camera is out of focus, or the image appears sharp, when the optical system is in focus.
In a second type of conventional range finder, the condition which must be determined is the degree of alignment between two different portions of the range finder image. In this type of range finder, bringing the optical system into focus causes the portions of the image to be brought into exact alignment, thereby causing the previously non-aligned, i.e., split image, to become a single image indicative of a properly focused condition. When the lens is adjusted to align the bottom and top portions of the image (i.e., to bring the two double images into exact registration) the in-focus condition is thus realized.
As above indicated, systems for providing automatic focusing of the optical system of a camera have also been disclosed in the prior art. One type of arrangement based upon the formation of a double image is disclosed in U.S. Pat. No. 3,511,156 which issued on May 12, 1970 to Leonard Larks. In the aforesaid arrangement, double images are formed at an image plane and are purposely separated along a direction orthogonal to the direction along which they are separated due to the out of focus condition of the lens system. The images are then both caused to scan the image plane in the latter separation direction. Due to their separation, one image begins and ends its scan before the other image commences to scan. Light detectors are provided to respond to each image and to generate electrical signals which are displaced in phase by an amount which is indicative of the image separation. In turn, the image separation is indicative of the out of focus condition of the lens system. These electrical signals are then used to control a servo-mechanism which adjusts the lens system to reduce the phase displacement to zero, thereby bringing the lens system into focus.
Another known system for focusing an optical system using multiple images is disclosed in U.S. Pat. No. 3,784,305, issued to Rudolf Hartman on Jan. 8, 1974. In the system disclosed in the aforesaid patent, an image forming lens is placed in the path of the light rays emanating from an object and being imaged by an optical system onto the image plane. The lens is then rotated about its rear-most point so as to displace the front end of the optical axis. If the optical system is in focus, the rotation of the lens is found to have no effect on the image at the image plane. If the optical system is out of focus, the image at the image plane moves laterally. With this focusing system, the focus condition of the optical system is detected by the movement or lack of movement of the image being viewed in response to oscillation of the imaging forming lens.
The arrangements discussed above for detecting the focusing condition of an optical system have proven unsatisfactory under conditions during which the viewer can not accurately perceive when the image is no longer clear and sharp or when the split images are not precisely aligned.